The present invention provides a lightweight anodized magnesium or magnesium-alloy piston, and a method for manufacturing the same.
In order for a piston to function properly in an internal combustion engine, the piston must satisfy several requirements. First, the piston must be able to withstand the extreme temperatures (600-650 degrees Fahrenheit) associated with combustion in an engine. More particularly, the piston must be able to substantially maintain its shape after repeated exposure to extreme temperature and pressure. In other words, an ideal piston should exhibit limited xe2x80x9ccreepxe2x80x9d or distortion. Creep is a measure of how much a particular material distorts or moves (i.e. creeps) when exposed to intense heat and pressure, without returning to its original position after the heat and pressure are removed. Other desirable characteristics for a piston include good hardness and wear resistant properties.
In view of this criteria, pistons for internal combustion engines have typically been fabricated from aluminum or aluminum-based alloys. Piston motion causes vibration when the piston reciprocates in an engine, imposing side forces on the crankshaft. These forces are often balanced by expensive counterbalance systems in larger engines. As a result, lighter weight pistons which exhibit less vibration during operation of the engine are continuously being sought.
Attempts have been made to use lighter weight magnesium and magnesium-based alloys pistons. Magnesium pistons have been used in combustion engines with limited success at lower temperatures ranging from 400-500 degrees Fahrenheit; however, magnesium pistons are generally unable to maintain the required properties set forth above when exposed to the elevated temperatures (600-650 degrees Fahrenheit) more typically associated with combustion in an engine. More particularly, the magnesium pistons exhibit creep as the piston tends to shrink away from the cylinder bore over time. More recently, it has been determined that piston wear is also a problem with magnesium and magnesium alloy pistons.
In one aspect, the invention provides an anodized magnesium piston including a head and skirt for an internal combustion engine. The piston comprises a non-fiber-reinforced, magnesium-based alloy including up to about 4.0 percent by weight rare earth metals. The piston further comprises an external surface, at least a portion of which has a base layer of magnesium fluoride, magnesium oxofluoride, magnesium oxide or a mixture thereof electrochemically anodized thereto, and an internal surface.
In a second aspect, the invention provides a process of manufacturing a magnesium-based alloy piston including a head and a skirt for an internal combustion engine. The method comprises casting a piston using a non-fiber-reinforced, magnesium-based alloy including up to about 4.0 percent by weight rare earth metals. The piston has an internal surface and an external surface. The method further includes immersing the piston into an electrochemical bath containing fluoride ion, providing an electric current to the electrochemical bath, electrochemically anodizing a base layer of magnesium fluoride, magnesium oxofluoride, magnesium oxide or a mixture thereof to at least a portion of the external surface of the piston, and providing a low friction outer surface on the skirt.
In a third aspect, the invention provides a method of improving wear resistance and hardness of a piston for an internal combustion engine. The method comprises casting a non-fiber-reinforced, magnesium-based alloy including up to about 4.0 percent by weight rare earth metals to form a piston having an internal surface and an external surface. The method further comprises immersing the piston in a cooled, temperature controlled electrochemical bath including fluoride ion, providing an electric current to the electrochemical bath and electrochemically anodizing a layer of magnesium fluoride, magnesium oxofluoride, magnesium oxide or a combination thereof to at least a portion of the external surface of the piston. The anodized layer improves the wear resistance and hardness of the piston. In another aspect of the invention, this method further comprises employing the piston in a single-cylinder internal combustion engine in order to improve balancing and reduce vibration in the internal combustion engine when the engine is in use.